Fuse link



Nov. 9, 1948; E. H. YoNKERs FUSE LINK Filed April 26, 1947 Patented Nov.9, 1948 FUSE LINK Edward H. Yonkers, Glencoe, Ill., assigner to JoslynManufacturing and Supply Corporation, Chicago, Ill., a. corporation ofIllinois Application April 26, 1947, Serial No. 744,061

3 Claims.

The present invention relates to fuse links and more particularly toimprovements in fuse links of the character disclosed in applicantscopending applications Serial No. 514,030, filed December 13, 1943,Serial No. 533,824, filed May 3, 1944, and Serial No. 637,528, filedDecember 28, 1945, all assigned to the same assignee as the presentinvention.

It is an object of the present invention to provide an improved fuselink of sim'ple construction which provides long time overloadprotection, short time transient overload protection and isself-protected against damage due to lightning surges and the like.

In accordance with a further object of the invention, improved andexceedingly simple facilities are provided in the link for protectingthe link against burn-out or damage when subjected to lightning surgesor the like.

The present invention, both as to its organization and method ofoperation, together with further objects and advantages thereof, willbest be understood by reference to the following specification taken inconnection with the accompanying drawings in which the single figure isa fragmentary sectional view illustrating a fuse link characterized bythe features of the present invention.

Referring now more particularly to the drawing, the present improvedfuse link 5 is there illustrated as comprising a tubular metal casing 3|having an open lower end and a flanged upper end which is closed by aterminal cap 32.

The construction and mode of operation of the fuse link componentsprovided within the metallic casing 3| are substantially the same asdisclosed and claimed in applicants co-pending application Serial No.637,528, iiled December 28,' 1945, and assigned to the same assignee asthe present invention, with the important exceptions pointed out below.This casing 3| houses three fusible elements 40, 4| and 42 which areconnected in series circuit relationship between the casing 3| and thepigtail conductor 37. The two elements 4| and 42 are both formed ofNichrome wire or another metal wire or ribbon of the desired resistivityand are both of the same cross sectional area, such that they possesssubstantially identical time-current fusing characteristics. The fusibleelement 4| is substantially straight throughout the major portion of itslength and the lower end portion thereof is centrally disposed Withinthe upper end portion of a stranded pigtail conductor 31, theoverlapping portions of the two elements 4| and 3l being telescopedWithin a (Cl. 20D-115) metal assembly sleeve 44. This sleeve is crimpedadjacent the lower end thereof, as indicated at 44a, and the lower endof the fusible ,element 4| is brought out through the strands of theconductor 3l and wrapped around the conductor 31 as indicated at 4|b.After the three elements 4|, 3], and 44 have been assembled to occupythe relative positions illustrated, the crimp 44a may be formed aroundthe lower portion of the sleeve 44 for the purpose of providing a rigidmechanical connection between the three named parts. Thereafter, thelower end portion of the sleeve 44, the adjacent portion of the pigtailconductor 3l and the wrapped end portion 4|b of the fusible element 4|may be soldered to provide a rigid connection therebetween.

The fusible element 42 may more properly be designated a combinationimpedance element and heating element. in that it functions inconjunction with theimproved surge gap facilities described below toprevent surge currents of large magnitude from traversing itself andalso acts to heat the fusible element to a fusing temperature when thelink is subjected to an overload current for a sustained time interval.In order to perform these two functions the element 42 ls constructed inthe form of a helically coiled conductor, and the upper end portion 42athereof is electrically and mechanically connected to the under side ofthe contact head 32 by crimping the same between the flange 3 la and thecap 32.

The convolutions of the element 42 are spaced apart axially of the tube3|, and the spaced apart relationship between the turns is maintained'by embedding the same in Ia body of dielectric refractory material 45.This ibody is preferably formed of a refractory cement and servesseveral functions which are pointed out with particularity below. Itmay, for example, Ibe formed of Portland cement or any ceramic materialwhich is chemically inert, has high specific heat, and is possessed ofgood electrical insulating properties. In order to increase theinductance of the element 4'2, thereby to enhance the surge currentblocking function thereof, particles of magnetic,

material, such, for example, as iron powder or magnetite, may bedisposed throughout the body 45, but in no case should the density of.the magnetic particles be such as to provide conductive paths capableof short-circuiting the convolutions of the element 42. Among otherfunctions, the body 45 serves rigidly to position or support the lturnsof the element 42 within the tube 3|, and to this end entirely fills theupper portion of the tube. It also serves to support a tubular conductord@ centrally oi the tube iii, this element being `utilized in theconnection. oi the fusible element il with the lower end portion o thecoinn hination heating and inductance element [ith lviore specifically,the upper tubular portion .the connecting element it is projected wellwithin the turns of the element in spaced apart relationship therefrom,and is embedded in the it its lower end this connecting element isprovided with an outwardly extending flange [lilo which serves to seatthe lower turn of the element l2 in a manner such that the tubularportion oi the element it is substantially concentrioally dis= posedwithin the turns of the element fill. This lower turn o'i the element t2is electrically and mechanically connected to the flanged portion bitoor the element it |by means oi a high iinciting point solder dl, or thelike. The upper end of the connecting element d is electrically andmechanically connected to .the upper serpentine end ta of the fusibleelement fil through the fusible element do, the latter element being inthe iorm of a body of alloy solder having a melting point ofapproximately 365 F. lt is to be noted that the fusible element do, asthus formed within the tubular portion oi the connecting element 46, isdisposed well Within the turns oi the element t2 so that heat generatedby current conduction through :the latter element may be transferred tothe fusible element lo through that portion of the refractory body liliwhich separates the fusible element Il@ from the adjacent turns of theelement [52. Spring rtension imposed upon the free lower end of thepigtail conductor till may be utilized to rapidly withdraw the end liaoi the fusible element di from the connecting element d@ when the namedfusi-ble element is heated to a melting temperature, and to widen aibrealr in the fusible element di occasioned 'by heating this element.to a fusing temperature.

For the purpose oi increasing the heat storage capacity ci the structureincluding .the fusible element du, a thermal storage element is pro=vided which is arranged in heat transfer relationship with the elementdii and the heating element rlihis element is in the form oi a copper orbrass rod disposed centrally 0i the tubular casing iii and having itslower end con tacting the exposed upper suriace of the iusible elementfill., lt is held in an upright position in axial alignment with thetubular conductor do by virtue of its being embedded in the body Duringprolonged use oi the link, .the upper por1 tion of the fusible elementiii may many times be heated to a fluid state without actual rupture oithe element due to lacie of persistence of the current overloadresponsible for the excessive heat energy. Also repeated heating landcooling of the body dii may result in the formation or small ntersticestherein through which the fluid metal could be dispersed toshort-circuit the lower convolutions of the coiled conductor t2. Fthiswould result in undesired modification of the tinten-overloadcharacteristic of the linie. In order to obviate this possibility, aceramic insulating tulbe de is provided as a barrier between the partslili, tti, and it and the turns of the coiled conductor element ft2.Specifically, this tube is telescoped over the tubular conductor Citiyto rest upon the flange ita and is embedded in 'the body d5 in themanner illustrated. Being oi heat resistant ceramic material, the tubedii is not subject to cracking and hence acts as a leali-prooi barrierbetween the fusible element dit and the convolutions or the coiledconductor element di. Als-o, since the tube it has approximately thesame heat transfer characteristics as the -body its presn ence in thezone oi heat transfer between the ele ments iii and itl and the elementlf/2 does not seriously complicate the `problem of producing linirshaving substantially the same timemverload characteristics in productionquantities. Further, provi-sion of the tube la in the structurefacilitates assembly of the component elements oi the structure in themanner explained below.

ln order to maintain the turns or the element it out of contact with themetal casing 3i, to maintain the element di out of contact with thetubular casing 3l and to provide a support for the surge gap facilitiesdescribed below, the entire assembly within the casing @l and a part ofthe sleeve 48 are surrounded by a tube itil formed or Bakelite or othersuitable insulating material. At its upper end, this tube is providedwith a ange 5Ela pressed against the metal casing flange 3io when thecap 32 is fastened to the ange Bia. This tube 5i! fits snugly within andis adhesively 'bonded to the tubular member si, and the lower endportion 50o thereof projects outwardly from the open lower end of thecasing 3i. It is counn terbored from the upper end to provide a lrstportion 5de of large internal diameter for rreceiving the parts lit, to,fili, ond portion till; of smaller internal diameter ier receiving thefusible element di. The flange dta oi the 'tubular conductor M is seatedupon the step bild between these two portions of the tube 5i).

ln order to assist in producing arc extinction within the tube @il whenthe linie is ruptured either through fusing oi the element lil or :usingof the element Lil, that part oi the tube portion. obb through which thefusible element di i xtends may be lined with a gas evolving materialdi., Preferably, this lining is in the torni of a layer of long nbercellulose adhesiifely secured to the inner surface o the t be portionhlibbetween the flange bild and the upper end oi the sleeve (irlsurround the fusible element lli,

ln accordance with the present invention, in order to prevent lightningsurges irorn rupturing the fusible heating element di?, a surge gaphaving relatively immovable electrodes 'is connected shunt with thiselernenu Specifically, the heat storage element id having lower end. inelecw trieal and mechanical contact with the upper ci the fusible bodytil, is provided with an up er end which is spa-coil apart iroro amember it associated with the cap assembly in a manner so as to deilne asmall gap between the member 5t and the adjacent end oi the heat storagemember Lid. This gap in combination with the storage element and thefusible body provides a surge path in shunt with the heating element il?for toy-passing surges around this element.

lin considering the inode ot operation of the iuse linie i5, lit may beassumed that the transformer which the link protects is provided with alow voltage secondary load which under normal conditions approximatesthe full load capacity ot the transformer, and that this secondary loadincludes motors and other devices which, during the starting periodsthereof, are capable of prou ducing heavy transient currents ofrelatively short duration in both the primary and secondary windings oithe transformer. In this regard, it will be understood that due to theheat radiating capabilities of the transformer parts, the transformerdit and 49, and a secmay be capable of withstanding an overload currentof reasonable magnitude, such, for example, as 200 to 300 per cent, fora relatively long time interval; whereas it can withstand current of theorder of 500 per cent of normal for only a short time interval. It willalso be understood that the transformer is capable of being damaged by asustained increase in the voltage applied thereto.

'I'he fuse link I5 operates to protect the transformer against damageoccasioned by overload currents caused by any one of the mentionedfactors. At the same time, the fuse link permits the transformer to beoperated under sustained overload current conditions for a period lessthan that required to damage the transformer, and will not rupture whensubjected to the normal and non-injurious high currents which areproduced incident to motor starting, or the like. In this regard it willbe understood that since the three serially related elements 40, 4| and42 of the link I5 are traversed by the current flowing through theprimary winding of the transformer, they are all heated by currentconduction and the temperature of each element varies with changes inthe magnitude of this current. The fusible element 40 is also heated bythe heat conducted thereto from the element 42 through the walls of theconnecting element 46. Heat energy is also transferred from the turns ofthe element 42 to the fusible element 40 through that portion of therefractory body 45 and the tube 49 which are disposed between thetubular portion of the connecting element 46 and the surrounding turnsof the element 42, and also from the thermal storage element 48. Underconstant load current conditions and with a constant voltage so long asthe load current through the secondary winding of the transformer doesnot substantially exceed the full load capacity of this transformer, thefusible element 40 is operated at a temperature well below that requiredto produce fusion thereof. When, however, the secondary load current ofthe transformer gradually rises and is sustained for a periodapproachingJr that at which the transformer will be damaged, thetemperature of the fusible element 4U is raised accordingly. Thus, asthe load current increases, the current traversing the three elements43, 4I and 42 is correspondingly increased so that more heat is producedin the fusible element 4U by current conduction. Concurrently the amountof heat conducted to this fusible element from the fusible elements 4|and 42 is increased. Also concurrently, the amount of heat transferredfrom the turns of the element 42 to the fusible element 40 through therefractory body 45 and the tube 49 is increased. A portion of the heatenergy accumulating in the element 40 is transferred to the storageelement 48 by conduction. After a predetermined time interval, requiredfor the accumulation of heat within the fusible element 23, this elementis heated to its fusing temperature and melts. Provision of the storageelement 48 materially increases this time interval over that which wouldobtain in the absence of this element in the combination. When theelement 40 is thus ruptured, the fusible element 4|, the sleeve 44, andthe upper end of the pigtail conductor 31, are quickly expelled from thelower end of the tube 50, thereby rapidly to break the circuit forenergizing the primary winding of the transformer.

During the described separation of the element 4I from the tubularconductor 46 upon fusing of the element 4U, an arc is drawn within theportion 50h of the tube 50. This arc instantly heats the coating BI togas evolving temperature, with the result that gases are evolvedtherefrom which accelerate extinction of the arc. It has been found thatprovision of the gas evolving coating 5I permits the link itself toeffectively extinguish heavy current arcs of the order of severalhundred amperes even when the link is used in high voltage circuits ofseveral thousand volts.

To consider the action of the fuse link I5 further, it may be pointedout that the refractory body 45 and the tube 49 prevent the fusibleelement 40 from being heated to its melting temperature when surgecurrents are produced in the primary circuit of the transformer as aresult of motor starting or the like. Such 'surge currents are of shortduration, being of the order of only a few seconds. The resultingmomentary increase in heat generation within the fusible element 4I) iswholly insufficient to raise the temperature of the element to itsmelting point. Moreover, those portions of the refractory body 45 andthe tube 49 which are disposed between the tubular portion of theconnecting element 46 and the storage element 48 and the turns of theheating element 42 dissipate a large portion of the heat resulting fromthe current surge through the element 42 away from the element 4I. Theyalso delay the transmission of the increased increment of heat producedby the element 42 to the storage element 48 and the adjacent walls ofthe connecting element 4E for an interval which will normally exceed theduration of the current transient. Accordingly, the increased incrementof heat energy arriving at the surfaces of the connecting element 46 andstorage element 48 from the element 42 as a result of the momentary highcurrent, effects an insufficient increase in the temperature of thefusible element 40 to cause this element to/melt. In other words, thetotal heat accumulated in the fusible element 40 as a result of thetransient high current is insufficient to heat this element to itsfusing temperature. Thus it will be understood that the refractory body45 and the tube 49, or more exactly the thermal impedance of thiscomposite structure, protects the fusible element 40 against outagesoccasioned by transient currents of the character which frequently occurin the load pattern of any transformer secondary load. This isaccomplished, moreover, Without increasing the thermal capacity of thefusible element 40 to a point such that it will provide no protectionfor sustained overload currents.

The thermal impedance of the refractory body 45 and the tube 49 alsoprevents the fusible element 40 from immediately rupturing when thetransformer is subjected to a high current, such, for example, as thatwhich is produced when the secondary winding of the transformer isshortcircuited. In the absence of an additional protective element,therefore, the transformer could easily be damaged by an overloadcurrent of this character during the period required to transfersuiilcient heat from the heating element 42 to the fusible element 40 tocause the latter element to melt. The second fusible element 4|functions to guard the transformer against damage when subjected to anoverload current of this type. Thus, immediately the element 4| issubjected to a transient current of the particular character justreferred to, a portion thereof lying between the upper end of the sleeve44 and the lower end of the fusible element 40 is heated to a fusingtemperature, permitting this element to rupture.

While there has been described what is at presandasse t? ent consideredto be the preferred embodiment of the invention, it vviil be understoodthat vari ous modications may be made therein which are within the truespirit and scope oi the invention as defined in the appended claims.

i claim:

l. A fuse link comprising a tubular metal cas ing open at one end andhaving a terminal can closing the other end, an insulating tube fittingWithin and extending through said casing, a coiled heating conductordisposed Within said tube and having its convolutions spaced axially ofsaid tube, a tubular conductor extending within the convolutions of saidcoiled conductor to receive heat from said coiled conductor, saidheating con-7 duct-or being connected in series between said tubularconductor and said casing, a fusible body filling at least a portion ofsaid tubular conductor, a circuit conductor having an end embedded insaid fusible body 'and extending out of said insulating tube, anelongated and conductive heat storage element in heat transferrelationship with said fusible body and extending through theconvolutions' of said heating conductor toward said terminal cap, meansincluding said'heat storage element for defining within said tube asurge gap shunting said heating conductor, and insulating means disposedwithin said insulating tube to insulate said tubular conductor and saidheat storage element from the convolutions oi said heating conductor.

2. A fuse link comprising a tubular metal casing open at one end andhaving a terminal can closing the other end, an insulating tube iittingwithin and extending through said casing, a coiled heating conductordisposed Within said tube and having its convolutions spaced axially ofsaid tube, a tubular conductor extending within the convolutions of saidheat from said coiled conductor, said heating conm1 ductor beingconnected in series between said tubular conductor and said casing, afusible body iilling at least a portion oi said tubular conductor, acircuit conductor having an end embedded in said fusible body andextending out of said insulating tube, an elongated and conductive heatstorage element in heat transfer relationship with said fusible body andextending through the couvolutions or said heating conductor toward saidcoiled conductor to receive' lil `tubular conductor and saidconvolutions of said coiled terminal can, said heat storage elementbeing' electrically connected with said fusible body one end and havingits opposite end spaced from said terminai can to define a surge gapshunting said heating conductor, and insulating means disn posed Withinsaid insulating tube to insulate said heat storage element from theconvolutions oi said heating conductor.

3. A fuse link comprising a tubular rnetal casing open at one end andhaving a terminal cap closing the other end, an insulating tube fittingWithin and extending through said casing, a coiled heating conductordisposed Within said tube and having its convolutions spaced axially ofsaid tube, a tubular conductor extending within the conductor to receiveheat from said coiled conductor, said heating conductor being connectedin series between said tubular conductor and said casing, a fusible bodylling at least a portion of said tubular conductor, a circuit conductorhaving an end embedded in said fusible body and extending out of saidinsulating tube, a second tube of insulating material telescoped oversaid tubular conductor within said coiled conductor to insulate theconvolutions of said coiled conductor from said tubular conductor andsaid fusible body, and an elongated and conductive heat storage elementin heat transfer relationship with. said fusible body and cxtendingthrough said second tube toward said terminal cap, said heat storageelement being electrically connected with said fusible element at oneend and having its opposite end spaced from said terminal cap to dene asurge gap shunting said heating conductor.

EDWARD H. YONKERS.

REFERENCES (Cil'iilEllI The following references are of record in the ieof this patent:

UNITED STATES PATENTS Number

